Conventionally, various biochemical measurements have been performed by using test paper pieces impregnated with various reagents. These test paper pieces are wetted with biochemical substances, such as urine, blood, and so on, and the color changes in these test paper pieces resulting from various color reactions are evaluated visually or by photoelectric devices.
FIG. 6 is an enlarged view of an essential part of such a conventional photoelectric biochemical measuring instrument. This biochemical measuring instrument comprises a meter main body 72 which is internally incorporated with a reflection sensor 71 consisting of a light emitting element and a light receiving element, and a test paper cover 74 which is urged by a spring 73 towards the meter main body 72, defining an insertion passage 77 for receiving a test stick 76 consisting of a synthetic resin strip between the meter main body 72 and the test paper cover 74. On the test stick 76 is mounted a piece of test paper 75 impregnated with a suitable reagent. The reflection sensor 71, which is electrically connected to an electronic circuit unit, faces an opening 78 which is provided in the meter main body 72 for conducting light from the reflection sensor 71 to the test paper 75 and then back to the reflection sensor 71. A reference reflection plate 79 for measuring a blank value is fixedly attached to the test paper cover 74 opposite to the opening 78.
To perform a biochemical measurement such as a measurement of a urine sugar value, light is projected from the light emitting element of the reflection sensor 71 onto the reference reflection plate 79 as shown in FIG. 7, and the light reflected from the reference reflection plate 79 is received by the light receiving element of the reflection sensor 71 as a reading of a blank value or a reference value. Thereafter, sample, in this case urine, is impregnated in the test paper 75 of the test stick 76, which is then left as it is for a certain time interval required for the required color reaction. Then, the test stick 76 is inserted into the test stick insertion passage 77 which is opened up by sliding back the test paper cover 74 against the force of the spring 73. In this state, the face of the test paper 75 opposes the reflection sensor 71 via the opening 78, and the test stick 76 is pressed between the peripheral part of the opening 78 and the reference reflection plate 79 as shown in FIG. 8.
Here, light is projected from the light emitting element of the reflection sensor 71 onto the surface of the test paper 75, and the light reflected therefrom is captured by the light receiving element of the reflection sensor 71 for evaluating the urine sugar value from the intensity of the captured colored light by comparing the intensity of the captured light with the blank value and displaying it on a display unit.
However, this conventional reflection plate is generally planar, and is, in particular, provided with a planar reflection surface. Therefore, the reflection surface of the reference reflection plate will contact the reverse surface of the test stick which faces away from the test paper. Therefore, when an excessive amount of sample, which may consist of urine, blood or the like, is impregnated in the test paper, the sample could be deposited from the test stick onto the reflection surface of the reference reflection plate. Such a deposition prevents an accurate measurement of the blank value by altering the reflection property of the reflection surface, and substantially impairs the accuracy of the biochemical measurement.
Furthermore, since the test paper cover is required to be slid back before a test stick is inserted into the biochemical measuring instrument, handling is not very favorable.
Copending U.S. patent application Ser. No. 072,817 filed Jul. 13, 1987, discloses a photoelectric biochemical measuring instrument, and the disclosure of this prior application is hereby incorporated in the present application by reference.